Print a notice when 'install -j' is used inside a sandbox.

With 'install -j' it can be a bit hard to tell where the packages are
installed. This will be reworked once we get a unified dynamic UI for 'install'.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	cabal-install/Distribution/Client/Sandbox.hs
Merge remote-tracking branch 'origin/master'